ABSTRACT: We study sustainability of collusion with optimal penal codes, in markets where demand growth may trigger the entry of a new firm. In contrast with grim trigger strategies, optimal penal codes make collusion easier to sustain before entry than after. We compare different reactions of the incumbents to entry in terms of: sustainability of collusion, incumbent’s profits, entrant’s profits, consumer surplus and social welfare. Surprisingly, the incumbent firms may prefer competition to collusion.